1. Field of the Invention
The present invention relates to a ceramic honeycomb catalytic converter which can be suitably used for an exhaust gas clarification system of an internal combustion engine for vehicles.
More particularly, the present invention pertains to a ceramic honeycomb catalytic converter which comprises a metal casing, a ceramic honeycomb catalyst accommodated in the casing, and a retainer member in the form of a ceramic fiber mat disposed in a compressed state between an outer surface of the honeycomb catalyst and an inner surface of the casing, thereby generating a surface pressure for holding the honeycomb catalyst within the casing.
2. Description of the Related Art
As known in the art, ceramic honeycomb catalytic converters of the kind mentioned above include a ceramic honeycomb catalyst wherein a number of flow channels having a polygonal cell-like cross-section and extending longitudinally through the honeycomb catalyst are defined by a peripheral wall and partition walls arranged inside of the peripheral wall. Conventional arrangement of such ceramic honeycomb catalytic converters is disclosed, for example, in JP-A-57-56,615, JP-A-61-241,413, JP-A-1-240,715, JP-U-55-130,012, JP-U-56-67,314 and JP-U-62-171,614.
Such ceramic honeycomb catalytic converters have been widely spread primarily due to a high open frontal area of the ceramic honeycomb catalyst and a resultant low pressure drop when exhaust gas is passed through the flow channels in the honeycomb catalyst, making it readily possible to achieve an excellent exhaust gas clarifying performance. As a typical example, an advanced ceramic honeycomb catalyst used for practical purposes has a partition wall thickness or rib thickness of approximately 0.170 mm and a flow channel density or cell density of 60 cells per unit cross-sectional area of 1 cm.sup.2.
In accordance with a recent enhancement in the exhaust gas regulation as related to environmental problems, e.g., a requirement for reduction in the total emission amount of hydrocarbon in the LA-4 mode which is one of exhaust gas evaluation test modes in the United States, there is a strong demand for an improved ceramic honeycomb catalyst which is capable of achieving a distinguished exhaust gas clarifying performance as compared to conventional honeycomb catalysts. Specifically, in an operational state immediately after starting an engine, i.e., in the so-called cold start state, the exhaust gas clarifying efficiency undergoes a considerable deterioration because the catalyst is still not much warmed and hence it is not sufficiently activated. Thus, an early activation of the catalyst during the cold start state is considered as the most important task to clear the exhaust gas regulation. From such a viewpoint, as a general discussion, it has been proposed to reduce the thickness of the partition walls of the ceramic honeycomb structural body. The thin-walled ceramic honeycomb structural body serves on one hand to increase the open frontal area and thereby decrease the pressure loss and reduce the structure weight, and on the other hand to decrease the heat capacity of the catalyst and enhance the temperature elevation speed of the catalyst. In this case, a large geometric surface area of the honeycomb structural body can be obtained so that it is also possible to realize a compact structure. However, the thin-walled ceramic honeycomb structure, in turn, makes it difficult to achieve a predetermined minimum guarantee value, generally no less than 5 kgf/cm.sup.2, preferably no less than 10 kgf/cm.sup.2, of the isostatic destruction strength as one index of the structural strength. The term "isostatic strength" is defined in the JASO Standard M505-87, an automobile standard issued by The Corporation of Automobile Technology Association, Japan, and refers to a compressive destruction strength of the honeycomb structure under an isostatic or isotropic hydrostatic load, and is represented by a pressure value when the destruction occurs. Needless to say, ceramic honeycomb structural bodies with a poor isostatic strength require very careful handling, and may be readily subject to damage during the so-called "canning" process whereby the honeycomb catalyst is loaded into the converter casing and retained therein such that the honeycomb catalyst is prevented from dislocation due to vibrations, etc., which are encountered in practical use condition.
In many cases, the canning for retaining the ceramic honeycomb catalyst in place within a casing is effected by holding the outer peripheral surface of the honeycomb catalyst. However, the canning is sometimes effected in a different manner, e.g., by retaining the honeycomb catalyst solely in the exhaust gas flow direction, or in a combined mode in which the honeycomb catalyst is held at its outer peripheral surface while being retained in the exhaust gas flow direction. Normally, the canning is implemented using a ceramic fiber mat held compressed between the outer periphery of a honeycomb catalyst and the inner periphery of the metal casing, whereby the honeycomb catalyst is retained in place within the metal casing by a surface pressure generated by the ceramic fiber mat. In this instance, the catalyst canning structures, in particular the catalyst retainer members, are required to exhibit a high reliability in terms of the heat resistance. This is mainly due to the fact that, in view of the above-mentioned requirement for an early activation of the catalyst in the cold starting stage, the recent trend is to install the catalyst at a location close to the engine where the catalyst may be exposed to exhaust gas at a higher temperature, and/or to operate the engine under such a condition as to emit exhaust gas at a higher temperature. Emission of exhaust gas at a higher temperature may also result from an air/fuel ratio which is approximated to a stoichiometrical ratio in the high speed mode of the vehicle for satisfying various regulations regarding CO.sub.2 emission, fuel consumption, etc.
The requirement for a highly reliable heat resistance characteristic of the catalyst canning structures, in particular the catalyst retainer members, is also associated with a recent progressive application of the exhaust gas emission regulations to motorcycles, which necessitates an exhaust gas clarification system suitable for motorcycle engines. That is, due to a space limitation in the case of motorcycles, a catalyst converter is often installed within a muffler so that the metal casing with a catalyst converter housed therein is maintained out of contact with the open air and therefore hardly cooled. Consequently, the metal casing and the retainer member are subject to heating up to an extremely high temperature.
As a ceramic fiber mat forming the catalyst retainer member for the canning structure, it has been a general practice to use an intumescent, i.e., thermally expansive mat composed of alumina-silica fibers added with vermiculite. However, conventional intumescent mats proved to undergo deterioration in their compression characteristic, when they are heated beyond an upper limit temperature of 800.degree.-900.degree. C. More particularly, the surface pressure which had been acting to retain the honeycomb catalysts in place tends to decrease with the progress of deterioration. Then, it is no longer possible to stably retain the honeycomb catalyst in its initial position, so that the honeycomb catalyst tends to get premature wear as a result of friction with cone, retainer ring and/or end face cushion, etc., which are provided in the flow directional end region of the metal casing, or to be damaged due to intensive vibrations transmitted from the engines. Besides, the mats may scatter away when they are exposed to the intensive heat of exhaust gas. To overcome these problems, the ceramic honeycomb catalytic converter disclosed in the above-mentioned JP-A-61-241413 is combined with a ceramic fiber layer which is arranged between the intumescent mat and the inner surface of the metal casing. Such a solution, however, is not always appropriate because the resultant structural complexity makes it difficult to improve the manufacturing productivity of the ceramic honeycomb catalytic converters.
Besides, it should be noted that a reduced thickness of the partition walls of the ceramic honeycomb catalyst results inevitably in a decreased isostatic strength, and further that there may be instances wherein a thermal expansion of conventional mat rapidly increases the surface pressure generated thereby. The decreased isostatic strength of the thin-walled ceramic honeycomb catalyst in combination with the increased surface pressure may give rise to damaged to the ceramic honeycomb catalysts during their actual application. Thus, realization of a thin-walled ceramic honeycomb catalyst has been generally recognized to be practically incompatible with a stable retention of the honeycomb catalyst in place. To the knowledge of the inventors, there have been no proposals regarding the canning structure which is capable of stably retaining a thin-walled ceramic honeycomb catalyst in place for a long period.